The present invention relates to an adsorbent for arsenic species and a method and/or device for removing arsenite (As(III)) and arsenate (As(V)) from ground water and surface water systems. Particularly, the present invention relates to natural zeolite coated with nanophase Mnxe2x80x94Fe oxides (NZNPF), and a method and/or device for removing As(III) and As(V) from arsenic-contaminated waters using NZNPF.
High concentrations of arsenic in drinking water result from either anthropogenic contamination or weathering of naturally occurring subsurface materials. The two most important anthropogenic sources of arsenic are agricultural chemicals and wood preservatives. Furthermore, use of arsenic species in manufacturing of sulfuric acid and leather tanning have resulted in significant contamination. In the geological occurrence, arsenic is found at high levels in primary sulfide minerals, such as arsenopyrite (FeAsS), pyrite (FeS2), pyrrhotite (Fe1-xS), orpiment (As2S3) and realgar (AsS). In addition, its ionic and valence states allow arsenic to substitute in trace amounts for many elements in silicate rock-forming minerals (Onken and Hossner, 1996). The chemical similarity between arsenate and phosphate lead to anion solid solutions between such minerals arsenate pyromorphite (Pb5(PO4)3Cl) and mimetite (Pb5(AsO4)3Cl (Klein and Hurlbut, 1985).
In surface water systems, arsenic can be dissolved either directly from weathering of geological materials (Baker et al, 1998) or through mixing with high-arsenic geothermal waters (Wilkie and Hering, 1998). In groundwater systems, arsenic can also be derived from the dissolution of As-bearing iron oxides in unconsolidated aquifer materials (Korte, 1991). Aqueous arsenate concentrations are controlled by anion exchange and coprecipitation with Fe and Mn oxyhydroxides and are therefore a function of Eh and pH. Arsenate anion exchange dynamics are analogous to phosphate with competition for exchange sites generally favoring phosphate over arsenate. As(III) exists as a neutral As(OH)3 species and therefore is mobile under most conditions. Hence, As(III) has to be oxidized to As(V) which will result in formation of arsenate. The arsenate ion can then be adsorbed on to amphoteric Fe oxides.
In 1981, the US Environmental Protection Agency (USEPA) Region I office released a report examining the occurrence of elevated arsenic in the ground waters of Hudson, N.H. (EPA, 1995). Various government reports attributed the arsenic source to either a contaminant plume from either hazardous waste or pesticide source or the oxidation and release of arsenic from natural sulfide minerals, such as arsenic-containing pyrite or arsenopyrite in the bedrock aquifer materials. A similar problem was encountered in towns of Buxton and Hollis, Me. (Marvinney et al., 1994). There appeared to be no correlation with any particular geological unit although water from bedrock wells contained more arsenic than water from surficial wells. Furthermore, it was reported that 10% of the wells in the state contained arsenic  greater than 50 ppb. The US Geological Society (USGS) has recently compiled the results of arsenic water analyses throughout Massachusetts, Rhode Island, Vermont, New England, New Hampshire and Maine as a part of Water Quality Assessment (Ayotte et al., 1999). Of the 800 public and 190 private wells analyzed, 23% of the wells contained arsenic above the detection limit of 5 ppb and 10 to 15% of the wells contained  greater than 50 ppb arsenic levels.
As(III) in the concentration of 100 ppb is extremely toxic and may cause skin cancer and neurological damage. This is further illustrated in a recent report on arsenic poisoning in groundwater in India and Bangladesh. Tens of thousands in Bangladeshi villages are suffering severe health effects with skin legions, and warts and sores covering their hands and feet (New York Times, 1999).
Based on USEPA, the currently best available technologies for As(V) removal are summarized as follows along with their disadvantages listed in parenthesis: Ion exchange resins (sulfate concentration has to be  less than 50 mg Lxe2x88x921 to be effective; and resin is expensive); Activated alumina (performance is dependant on concentration of total dissolved sulfate and pH; and it is ineffective at high pH); Reverse osmosis (expensive; and concentrated waste stream is produced); Modified coagulation/filtration (it needs pre-oxidation for As(III) removal; and alum is ineffective at high pH); Modified lime softening (limited As removal at pH less than 10); Electrodialysis reversal (EDR) (expensive; creates large waste stream); and Oxidation/filtration (requires the use of Fe(II) to oxidize As(III), but the Fe to As ratio of 50 is critical for efficient removal). All these technologies are good only for As(V) and very little data is available for As(III). There is an urgent need to develop a low-cost method and/or device for the purpose of reducing As(III) and As(V) levels for supplying potable water in problematic areas.
The present invention provides an adsorbent for removing an arsenic species from contaminated waters. Particularly, the adsorbent is an Fe- and Mn-oxide. Representative examples include Mn-ferrihydrite, Si-ferrihydrite, Si-free birnessite, Si-birnessite, and natural zeolite coated with nanophase Mnxe2x80x94Fe oxides.
Specifically, for natural zeolite coated with nanophase Mnxe2x80x94Fe oxides (NZNPF), Mn-substituted nanophase Fe oxide removes both As(III) and As(V) from contaminated waters, wherein Mn(IV) oxidizes As(III) to As(V), and nanophase Mnxe2x80x94Fe oxide adsorbs As(V) and released Mn(II) and Fe ions. The removal is effective at all pHs (4 to 9) and the adsorption occurs in 30 min. Additionally, the amount of Mn and Fe released to water is decreased significantly, thus maintaining the aesthetics of water.
Also provided is a method for removing arsenic from contaminated waters by utilizing natural zeolite coated with nanophase Mnxe2x80x94Fe oxides. This method provides better water flow rate due to less oxides, which results in low headloss and reduced clogging of fixed bed filter media. For surface water treatment plants, zeolite assists in settling the particles in settling ponds.
Still provided is a filter system for removing arsenic from contaminated waters. Such filter system comprises a filter column with natural zeolite coated with nanophase Mnxe2x80x94Fe oxides (NZNPF), particularly, natural zeolite coated with 1% nanophase Mnxe2x80x94Fe oxides. After filtration, the resulting waters comprise less than 3 ppb of As(III) and/or As(V), which is below the instrument detection limit. This system may be used for treating both ground and surface waters.
Further provided is a method for removing arsenic having various valence states from arsenic contaminated waters by oxidizing the arsenites having lower valence states to arsenates having higher valence states; and then removing the oxidized and native arsenates having higher valence states from the waters. Other Mn-containing oxides than NZNPF, such as birnessite, Si-birnessite, and Mn-ferrihydrite, oxidize arsenite having lower valence states to higher valence states, and Fe-containing oxides, such as Si-ferrhydrite, and Mn-ferrihydrite, adsorb and subsequently remove the oxidized and native arsenates having higher valence states. This method may be used for treating both ground and surface waters.
Other Fe- and Mn-oxides may also be used for low-cost well-head filters for providing safe potable water. Such filter may be a single media filter or a dual-media filter. For dual-media filter, two absorbents are used: first adsorbent in the upper side of the filter column and second adsorbent in the bottom side of the filter column. The first adsorbent oxidizes As(III) to As(V), and the second adsorbent adsorbs the oxidized and native As(V) and released Mn(II) and Fe ions.
Additional objects, features and advantages will be apparent in the written description which follows.